Electrostatic spray painting, also referred to as powder painting, refers to a process wherein electrically charged paint particles are propelled by compressed air against a metal piece to be painted. The piece being painted is grounded so that it remains electrically neutral. The difference in electrical potential between the paint particles and the piece being painted causes the paint particles to adhere to the piece. The piece is then heated to more permanently affix the paint particles.
In a typical conveyor line painting process, the pieces to be painted are carried by the conveyor line through an electrostatic spray painting station which includes the painting equipment and heating equipment. The pieces to be painted are hung from metal hangers on the conveyor line, thereby electrically Founding the piece. The piece to be painted, and a portion of the hanger contacting the piece, travel through the painting station and are electrostatically spray painted. The painted piece and portion of the hanger are then heated to more permanently affix the paint particles to the piece. These painting and heating steps also result in paint particles becoming affixed to the portion of the hanger which travels through the painting station.
After the heating step, the painted piece is removed from the hanger for further conveyor work or packaging. Generally, the hanger remains affixed to the conveyor line and is used to carry additional pieces through the painting station. After several passes through the painting station, the portion of the hanger on which the piece is hung becomes coated with layers of paint. The build-up of paint prevents an electrically clean contact between the hanger and the piece being painted and, therefore, interferes with the proper grounding of the piece being painted. Problems arising in connection with insufficient grounding of the piece to be painted include bad turn in, uneven distribution of the paint a high consumption of paint and spark formation.
In order to overcome the problems associated with poor grounding of the pieces being painted, resulting from a build-up of paint on the hangers, a new hanger may be utilized for each painting operation. However, this solution is disadvantageous since the cost of new hangers would make many painting processes uneconomical and also because the need to hang new hangers on the conveyor line could interrupt the continuous nature of the painting process.
The disadvantages of paint build-up on hangers used in electrostatic spray painting is a problem which is well recognized in the art. For this reason, conveyor line operators periodically remove paint from the hangers in accordance with good housekeeping principles, and to forestall interference with line operation. However, the currently utilized procedures for removing paint from hangers can be expensive, cumbersome, slow and difficult.
For example, in one prior art method, paint built up on the hangers is removed by immersing the hangers in a tank of solvent to dissolve the paint. The labor to take down the hangers in this process is expensive. The solvents are likewise expensive and present disposal and and safety problems. Moreover, the dissolving process may require four to five hours. Since, it may be uneconomical to shut down the conveyor line for that period of time, it is necessary to keep a complete spare set of hangers to put on the line during cleaning, increasing both labor and equipment costs.
In order to overcome these problems, methods have been proposed for continuously cleaning the hangers used in an conveyor line electrostatic painting process, in a cleaning station forming part of the conveyor line. One such cleaning station is described in U.S. Pat. No. 3,830,196. In the system disclosed in the U.S. Pat. No. 3 196, hangers uses to suspend articles from a conveyor for transport through a continuous painting line are cleaned of accumulated paint by burning the paint to ash in an oven and spray washing to remove the ash. The burning and spray washing occur in a cleaning station located downstream of the electrostatic spray painting booth in the conveyor line. Unfortunately, a cleaning station such as the one envisioned by the U.S. Pat. No. '196 may be very energy consuming. In addition, the length of the furnace and consequently also the cost of its manufacture, is dependent on the speed of which the conveyor line travels and the size of the hangers on the line.
Another method for the continuous cleaning of hangers utilized in electrostatic spray painting processes is disclosed in U.S. Pat. No. 4,069,790. The process disclosed in the U.S. Pat. No. '790 utilizes heating filaments disposed within each of the hangers. The heating filaments are heated in a heating station located downstream of the electrostatic spray painting booth in order to cause residual paint left on the hangers to be burned off. Unfortunately, the preparation of hangers containing heating elements may prove uneconomical for many applications.
Accordingly, there remains a need for a system for cleaning the hangers utilized in an electrostatic spray painting process, which will clean the hangers in an economical fashion with minimal disruption to the conveyor line process.